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Conducting Maximal and Submaximal Endurance Exercise Testing to Measure Physiological and Biological Responses to Acute Exercise in Humans
Published on: October 17, 2018
Relationships between plasma volume, fluid regulatory hormones, and maximal oxygen uptake following fixed-workload
Andrew M Greenfield1, Shaun C Brazelton1,2, Billie K Alba1
1Thermal and Mountain Medicine Division, US Army Research Institute of Environmental Medicine, Natick, Massachusetts, United States.
Abstract:
Plasma volume (PV) expansion may support various adaptations associated with heat acclimation (HA), including a potential increase in temperate maximal oxygen uptake (V̇o2max). The PV response to HA is highly variable, and mechanisms that contribute to this variability are poorly understood, including the potential influence of sex. Twenty-six volunteers (11 M; 15 F) underwent a 10-day HA program that consisted of 120 min of fixed-workload treadmill exercise at 40°C and 40% relative humidity (RH). Temperate (∼20°C) V̇o2max and PV (carbon monoxide rebreathing) were measured before and after HA. Serum aldosterone, estradiol, and progesterone, as well as plasma copeptin, were measured on the first and last day of HA. HA was successfully induced, as evidenced by reductions in body core temperature, heart rate, and increased sweating rate during exercise in the heat. PV (Pre: 3,280 ± 552 mL, Post: 3,380 ± 685 mL; P = 0.096) and V̇o2max (3.06 ± 0.82 L/min, 3.09 ± 0.77 L/min; P = 0.453) were unchanged with HA at the cohort level, and sex did not modify responses to HA (interaction effects: P > 0.05). ΔAldosterone was correlated with ΔPV (P = 0.047, r = 0.393, r2 = 0.155) and Δprogesterone with ΔPV in women (P = 0.039, r = 0.537, r2 = 0.289). In our young, healthy cohort of men and women, PV and V̇o2max were unchanged after fixed-workload HA, and the overall adaptive response did not differ based on sex. The PV response was highly variable across individuals such that increases in PV were not observed in most individuals. Interindividual variability in circulating progesterone may contribute to some of the response in women.NEW & NOTEWORTHY We provide the first direct assessment of the plasma volume (PV) and volume regulatory responses to a practical, fixed-workload heat acclimation (HA) protocol in men and women. Maximal oxygen uptake was not influenced by HA, sex, or individual changes in PV. In women, progesterone was correlated with PV changes following HA.
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